Pyrroles that bear α-acyl groups (such as pyrrole-2-carboxaldehyde) are known to form coordination complexes with ketones (e.g., acetylacetone) and metals such as copper(II), nickel(II), palladium(II), or platinum(II).1-6 The typical reaction conditions employ the 2-acylpyrrole and the metal salt in refluxing ethanol. As part of a study aimed at developing a non-chromatographic method for purifying 1-acyldipyrromethanes (which contain the α-acylpyrrole motif), we screened a variety of metal reagents for formation of stable complexes of 1-acyldipyrromethanes. Dialkylboron reagents were found to serve exceptionally well as complexation aids in the isolation of 1-acyldipyrromethanes from the crude acylation mixture.7 In the course of this work, we made the surprising finding that some metal reagents, particularly those containing palladium and to a lesser extent copper, result in direct conversion of the acyldipyrromethane to the metalloporphyrin.
Palladium porphyrins are of interest owing to their high yield of intersystem crossing and long-lifetime of the resulting triplet state in diverse media. Palladium porphyrins have been used in diverse applications, including luminescent markers,8 oxygen sensors,9 sensitizers of singlet oxygen formation,10 and in photoinduced protein cross-linking.11 The first synthesis of PdTPP was reported in 1959.12 Palladium porphyrins are typically prepared by metalation of the free base porphyrin using Pd(OAc)2 in refluxing benzonitrile13 or in CH2Cl2/MeOH at room temperature.10 